One of the most visible direct impacts of fishing is the capture of non-target species, known collectively as by-catch. By-catch includes species that are unwanted and discarded (discards) and species that are retained and sold (byproduct) (Stobutzki et al., 2003). Although tuna purse seine fisheries have been shown to be selective, leading to lower levels of by-catch than other fisheries (Alverson et al., 1994; Kelleher, 2005), several species can be incidentally caught and, in some cases, discarded at sea. These include vulnerable and sensitive species such as turtles, mammals, and sharks (Minami et al. 2007; Romanov, 2008b). Tuna purse seine fisheries probably apply the most intensive direct human impact on the tropical epipelagic ecosystems in all oceans. Because of the worldwide scale of purse seine fisheries, an assessment of their impact on associated and dependent species is essential (Romanov, 2002a).